However, I've got good, consistent loads with both 208 Hornady HPBT's and 210 Berger VLD's, and neither will group at or under half MOA consistently.

It's not the shooter, bullet, or powder (Retumbo) that's responsible, and it certainly shouldn't be the gun! So that leaves my brass, and being the cheapest money can buy, I think it's a reasonable explanation.

TL;DR- Since I can't find any brass, can anyone tell me what factory ammo generally has the best, most consistent brass?

I ended up trying 300WM Norma brass which has been readily available at Sincairs. I had a hard time locating brass as well. I had not used it before and was pleasantly surprised. It's very uniform, I get superb accuracy and ES, and I've been able to get over 10 reloads out of it with an annealing and trim at five reloads. No pressure signs at all with 79gr of Retumbo and 210VLD's at 2880FPS. The only thing is that it's pricey, and the primer pockets are tight. You have to make sure they are well seated. They do not loosen up after multiple reloads though. I like the brass and am going to stay with it for my rifle.